1. Field of the Invention
The invention relates to a method and circuit for determining the speed of a rotating field or polyphase machine by monitoring the voltage induced in the stator by rotation of the rotor.
2. Description of the Prior Art
Devices for determining the speeds of polyphase asynchronous machines are available commercially, in which a converter can be connected to a rotating asynchronous machine. The frequency converter impresses on the asynchronous machine a reference sinusoidal current which may, for instance, be the rated current of the asynchronous machine. The converter traverses the frequency band of the asynchronous machine being measured by varying the current frequency from at D.C. and toward a maximum frequency. During this start-up process, the magnetic flux of the machine is monitored. As long as the frequency of the converter deviates from the frequency of the machine by more than the nominal slippage of the asynchronous machine, i.e., is outside the frequency band or window of the machine, the magnetic flux is very small. If the deviation between the converter frequency and the machine frequency becomes smaller than the nominal slippage of the asynchronous machine, the flux builds up at a flux time constant. The point at which the flux exceeds a predetermined limit, is used as an indication that the converter frequency and the machine frequency are approximately equal, i.e., the operating speed of the rotating polyphase asynchronous machine has been found. As soon as this criterion is met, the converter is switched to normal operation.
The search for the operating speed of the rotating asynchronous machine or a synchronous machine, is time consuming because a sufficiently large magnetic flux must be build up in the rotating-field machine while the converter traverses its frequency window. The predetermined value of the magnetic flux is exceeded only if there is sufficient time spent within the machine's frequency window for the flux to be built up. If the searching rate is increased, it may be possible that the frequency window is traversed too fast, whereby, sufficient flux cannot build up to exceed the predetermined magnetic flux value. Without exceeding this level, the operating speed of the rotating polyphase machine cannot be found and the converter cannot be connected to this machine, in order, for instance, to accelerate it again. In addition, the converter output voltage or the converter output current must have the same frequency and given the same phase as the polyphase machine when the converter is connected thereto. Since rotating-field machines usually have no tachometer generators, it is not possible to connect the converter to the machine at the operating frequency.